It is well established that surgical patients under anesthesia become poikilothermic. This means that the patients lose their ability to control their body temperature and will take on or lose heat depending on the temperature of the environment. Since modern operating rooms are all air conditioned to a relatively low temperature for surgeon comfort, the majority of patients undergoing general anesthesia will lose heat and become clinically hypothermic if not warmed.
There have been many attempts at making heated blankets and pads, including pads in the form of heated underbody supports, heated mattresses and heated mattress overlays for therapeutic patient warming. Therapeutic patient warming is especially important for patients during surgery. It is well known that without therapeutic intra-operative warming, most anesthetized surgical patients will become clinically hypothermic during surgery. Hypothermia has been linked to increased wound infections, increased blood loss, increased cardiac morbidity, prolonged ICU time, prolonged hospital stays, increased cost of surgery and increased death rates.
Over the past 15 years, forced-air warming (FAW) has become one of the “standard of care” for preventing and treating the hypothermia caused by anesthesia and surgery. FAW consists of a large heater/blower attached by a hose to an inflatable air blanket. The warm air is distributed over the patient within the chambers of the blanket and then is exhausted onto the patient through holes in the bottom surface of the blanket.
Although FAW is clinically effective, it suffers from several problems including: a relatively high price; air blowing in the operating room, which can be noisy and can potentially contaminate the surgical field; and bulkiness, which, at times, may obscure the view of the surgeon. Moreover, the low specific heat of air and the rapid loss of heat from air require that the temperature of the air, as it leaves the hose, be dangerously high—in some products as high as 45° C. This poses significant dangers for the patient. Second and third degree burns have occurred both because of contact between the hose and the patient's skin, and by blowing hot air directly from the hose onto the skin without connecting a blanket to the hose. This condition is common enough to have its own name—“hosing.” The manufacturers of forced air warming equipment actively warn their users against hosing and the risks it poses to the patient.
To overcome the aforementioned problems with FAW, several companies have developed electric warming blankets. Some of these warming blankets employ flexible heaters, the flexibility of which is desirable to maintain when employing the blankets. In many cases, an electric warming blanket employs a shell for holding the heater and for serving other purposes. For example, in some cases the shell includes layers formed of a substantially water impermeable material to help prevent fluid damage to the heater. Also, when these heaters are used for patient or other care, especially in the operating room, the shell can protect the patient and others in the vicinity from electric shock hazards. In addition to often providing a seal around the heater, the shell often contains a fastening mechanism that must reliably attach the heater to the shell to prevent electrical shorting across the heater during folding of the electric warming blanket.
Because the seals of the shell must be very reliable, the seals have traditionally been adhesive seals that are reinforced with combinations of sewing, rivets, and grommets. Sewing stitches, rivets, and grommets all share one characteristic—they all perforate the material layers to create a mechanical linkage between the layers.
While such a reinforced bond may be desirable for strength, it can create additional problems when used during surgery or medical procedures. For example, heated blankets placed over a patient during a surgery or medical procedure are frequently soiled with waste blood or other body fluids. The fluid waste can saturate the stitching and then dry and accumulate in the thread or the stitch holes. If rivets or grommets are used for reinforcement, additional crevasses are introduced that can trap waste fluids. When the outer shell of the blanket is cleaned by hospital personnel, it is nearly impossible to clean the residual contaminating materials out of the holes, crevasses, and/or stitches. Therefore, the stitching holes and thread, the grommets, rivets and snaps can all become sources of microbial contamination because they cannot be thoroughly cleaned and disinfected.
Prior to the 1990's, warm water mattresses were commonly used. The warm water mattresses went out of common use because they were relatively stiff and inflexible. The stiff water mattress negated any pressure relief that the underlaying support mattress may have provided. As a result, the combination of pressure applied to the boney prominences and the heat from the warm water mattress both reduced blood flow and accelerated metabolism, causing accelerated ischemic pressure injuries to the skin (“bed sores”). Additionally, the warmed water recirculating in the warming system was well known to be grossly contaminated with bacteria, which was especially important when a leak occurred. As a result, warm water mattresses are rarely used today.
Historically, electrically heated pads and blankets for the consumer market have been made with resistive wire heaters. Wire-based heaters have been questionably safe in consumer use. However, in the operating room environment with anesthetized patients, hot spots caused by the wires in normal use and the failure mode of broken heater wires resulting in sparking, arcing and fires are totally unacceptable. Therefore, resistive wire-based heaters are not used in the operating room today.
Since the mid 1990's, a number of inventors have tried unsuccessfully to make effective and safe heated mattresses for operating room use, using flexible, sheet-like electric resistance heaters. The sheet-like heaters have been shown to be more effective in warming the patients because of the even heat production and generally do not cause arcing and sparking when they fail.
Some existing devices employ sheet-like heaters using a polymeric fabric that has been baked at high temperature until it becomes carbonized and is thus conductive of electricity. The carbonization process makes the fabric fragile, and therefore, it may be laminated between two layers of plastic film or fiber-reinforced plastic film for stability and strength. The lamination process results in a relatively stiff, although somewhat flexible, non-stretching, non-conforming heater. The metal foil bus bars are attached to the heater material with an “electrically conductive adhesive or bonding composition . . . ” and then encapsulated with polyurethane-coated nylon fabric. The result is a stiff and relatively inflexible bus bar.
Clearly, there is a need for conductive fabric heaters for use in therapeutic heated mattresses that are highly flexible, stretchable in at least one direction and durable without needing lamination to stabilize or protect the heater fabric. There is also a need for bus bar construction that does not result in thick, stiff, inflexible areas along the side edges of the heater. Then, maximally effective and safe therapeutic heated mattresses need to be designed using the stretchable, durable fabric heaters.
In addition to patient warming during surgery, and as known to those skilled in the art, modern surgical techniques typically employ radio frequency (RF) cautery to cut and coagulate bleeding encountered in performing surgical procedures. Every electrosurgical generator system may have an active electrode that is applied by the surgeon to the patient at the surgical site to perform surgery and an electrical return path from the patient back to the generator. The active electrode at the point of contact with the patient may be small in size to produce a high current density in order to produce a surgical effect of cutting or coagulating tissue. The return electrode, which carries the same current as the active electrode, may be large enough in effective surface area at the point of communication with the patient such that a low density current flows from the patient to the return electrode. If a relatively high current density is produced at the return electrode, the temperature of the patient's skin and underlying tissue will rise in this area and can result in a patient burn.
Return electrodes have evolved over the years from small 12×7-inch, flat stainless steel plates coated with a conductive gel that were placed under the patient's buttocks, thigh, shoulders, or any location where gravity could ensure adequate contact. The next development was flexible foam-backed electrodes. These flexible electrodes are about the same size as the stainless steel plates and are coated with a conductive polymer. They have an adhesive border so that they remain attached to the patient without the aid of gravity.
Described as early as 1938 and first introduced into the surgical market in 1960, capacitively coupled return electrodes offer an alternative to conductive return electrodes. Unlike conductive electrodes, which involve direct patient contact, a capacitively coupled electrode is placed close to, but not touching, the patient. It is separated from the patient by a dielectric barrier—that is, a layer of insulating material. This allows the electrode to form a capacitor with the patient. A capacitor is an electrical circuit element used to store a charge temporarily. In use, this type of electrode induces a current flow across the electrode-patient capacitor such that electricity is safely returned from the patient to the electrosurgical unit across a dielectric insulator layer, allowing the desired surgical effect at the surgical site.
A capacitively coupled return electrode consists of a single conductive plate, fabric or film that is encased in a dielectric material. The insulating material does not permit the charge to flow through the electrode to the patient. When placed in close proximity to each other, the conductive plate and the patient become capacitively coupled. Their separation is maintained by the electrode's insulating material, which forms a dielectric barrier between them. For example, a large flat sheet of conductive material that covers a portion of the operating table may be the electrode and the dielectric barrier may consist of plastic film, linens, cushions or other materials that may be placed between the patient and the electrode.
When the active electrode is applied at the surgical site, the electrosurgical unit induces an oscillating radio frequency (RF) voltage through the surgical site and between the patient and the return electrode's conductive plate. As this occurs, several events take place simultaneously. First, an electrical charge accumulates and diminishes in cycles, both on the surface of the patient over lapping the return electrode and on the electrode's capacitive plate, in equal and opposing polarities. Second, the dielectric material becomes polarized: an electrical charge will not move through it. Finally, as the electrical charge moves to and from the surface of the patient's skin, there is a loss of energy that produces a minimal amount of heat within the skin (as happens with a conductive return electrode).
If the dielectric is thin, meaning that the patient and the return electrode are close together—for example less than 2 mm—the capacitive coupling is very efficient. If the distance between the patient and the electrode increases, the efficiency of the coupling decreases. Therefore, minimizing the distance between the patient and the electrode may be desirable. The ability of this design to minimize the distance of both the heater and the grounding electrode from the patient may be particularly desirable with small pediatric patients who have minimal surface area contacting the support surface.
There is some concern that an unnoticed, accidental hole in the electrode's dielectric material could provide a conductive contact with the patient over a very small area, causing a large concentration of current to flow in a small area and to burn the patient. In some cases, thick layers of “self-sealing” gel material have been interposed between the electrode and the dielectric material to prevent a conductive pathway from occurring in the event of a hole in the dielectric material. The gel material is heavy and cumbersome.
Capacitive coupling electrodes generally have been mattress overlays, which are inconvenient, involving extra cleaning Additionally, they are usually non-stretching conductive fabric—for example, woven nylon embedded into a heavy, cumbersome gel pad—which reduces the effectiveness of the pressure-reducing mattress of the surgical table. The conductive silver coating on the fabric electrode also diminishes radiolucency to x-rays, causing x-rays that are shot through the mattress to be grainy or distorted.
The location of the capacitive coupling grounding electrode under the patient is in direct competition for space with heated underbody warming pads and mattresses commonly used in surgery. Heated underbody warming pads and mattresses also work optimally when in close contact with the patient's skin. Therefore, both of these safety technologies may not perform optimally when used simultaneously as two separate devices since seemingly only one or the other can be optimally placed adjacent the patient's skin.
Clearly, there is a need for improvement by combining the capacitive coupling electrode with the heated underbody warming system. However, simply combining the two technologies into a single shell could produce a laminated structure that would be less stretchable, less flexible and less accommodating—further preventing the patient from sinking optimally into the support mattress and increasing the risk of pressure ulcers.
Combining the capacitive coupling electrode with the heated underbody warming system in a single layer of stretchable, flexible material that can serve as a heater and grounding electrode simultaneously would prevent the problems resulting from a two-layer laminate structure and would reduce the cost and complexity of manufacturing.
Accordingly, there remains a need for heated blankets, shells and pads for flexible heaters that are readily and thoroughly cleanable. There also remains a need for improvements in electrosurgical grounding for surgery. In particular, there is a need for devices including these features that also offer pressure relief and prevent bed sores.
Various embodiments of the invention described herein solve one or more of the problems discussed above in addition to other problems that will become apparent.